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Strength, fatigue resistance and the evaluation of cyclic cracking resistance of new deformable aluminium alloys, alloyed with transition and rare earth metals.

1. Introduction

One of the current tendencies in the development of new aluminium alloys with high mechanical properties is the alloying of these materials with transition metals (TM) and rare-earth metals (REM) and optimisation of the thermomechanical treatment ((TMT) of these alloys [1-5]. According to the results obtained in these studies, the tensile strength of the new aluminium alloys may equal 800 MPa. However, there are insufficient data on the elasticity modulus, fatigue strength and cyclic cracking resistance of these materials. Therefore, the aim of the present work is the investigation of the characteristics of elasticity, fatigue strength and evaluation of the cyclic cracking resistance of new aluminium alloys with higher mechanical properties.

2. Experimental materials and procedure

The investigations were carried out on two groups of deformable aluminium alloys, additionally alloyed with TM and REM with different ratios, namely: group 1--the alloys of the system 5XXX (Al-Mg), i.e., alloys strengthened by the formation of the deformation structure and not subjected to strengthening by heat treatment; group 2--alloys of the system 7XXX (Al-Zn-Mg-Cu) which strengthen in heat treatment by the precipitation of the hardening phases. The chemical composition of the investigated alloys is shown in Table 1.

The ingots were produced in heating equipment consisting of a high frequency generator with an induction coil for heating the metal, a vacuum chamber in which the metal is melted and cast, a system for suspending the crucibles, a system for pumping and the supply of the inert gas. The system for mixing of the liquid metal, a copper ingot mould and a system for measuring the temperature of liquid metal, using chromel-alumel thermocouples were also used. Melting was carried out in ceramic crucibles. The metal was cast by bottom casting: into a water-cooled copper mould with water cooling and the riser part.

For additional purification, the melts were cast through the ceramic filters and the melt was blown with argon.

The complexly alloyed alloys were produced using the master alloys of the following composition: Al-37 wt.% Mg; Al-38 wt.% Zn; Al-50 wt.% Cu; Al-4.75% Zr; Al-2 wt.% Sc; Al-5 wt.% Cr; Al-5 wt.% Nb; Al-5 wt.% Mn; Al-5 wt.% Hf. Aluminium of grade A95 and A97 was used for the preparation of the charge.

The ingots with a diameter 55 mm were deformed to produce bars with a diameter of 6 mm by extrusion. Prior to heating, the ingots were machined in order to remove surface defects. The temperature of heating of the ingots and the pressing mould was 300-450[degrees]C for the alloys of the Al-Mg system, and 400-450[degrees]C for the alloys of the Al-Zn-Mg system. The extrusion temperature was 350-450[degrees]C, the pressure in extrusion of the ingots was varied in the range from 0.85 to 1.25 MN, depending on the composition of the alloys.

The appearance of the fracture surfaces, structure and distribution of the alloying elements with respect to the structural components were investigated in a scanning electron microscope with an x-ray analyser Superprobe 733. For consecutive analysis of the fracture zones, photographs were taken at x30, 300, 1000 and 2000 of the fracture surfaces of the specimens after the fatigue test.

The mechanical properties of the specimens were determined in the tensile test of fivefold specimens with a diameter of the gauge part of 3 mm. The tests were carried out with recording the deformation curve in the coordinates load P-elongation l. The strain rate was [10.sup.-3] [s.sup.-1]. The deformation curves were used to calculate tensile strength [[sigma].sub.B], yield limit [[sigma].sub.0.2] and the relative elongation to fracture [delta], %.

The elasticity modulus E (Young modulus) was determined using the procedure described in detail in [6] by the excitation of longitudinal oscillations in the investigated specimens. The natural frequency in the first form of these oscillations was fixed and elasticity modulus was calculated from the equation

E = 4r[l.sup.2] [f.sup.2], (1)

where l is the length of the bar specimen, f is the frequency of the natural longitudinal oscillations, p is the density of the material of the specimen.

The fatigue test was carried out in VEDS-200 electrodynamic test stand using the procedure described in detail in [7]. The test diagram is shown in Fig. 1. The scheme is characterised by the coefficient of asymmetry of the load (stress ratio) R = -1. The diameter of the experimental specimens was 3 mm, the length l = 70 mm, the surface of the experimental specimens was polished to a surface finish of 0.63 [micro]m. The fatigue test of the specimens produced from the alloys of the Al-Mg-Cu system was carried out after heat treatment of the specimens in the conditions T1.

The maximum stress in the critical cross-section of the specimen [[sigma].sub.a] was calculated from equation (2) on the basis of the resonance frequency [f.sub.i] of the specimen-beam system in the second form of the bending oscillations, the length of the cantilever part of the specimen [l.sub.k] to its diameter D, the coordinate of the fracture area x, the elasticity modulus E, the density of the materials investigated specimen and the amplitude of the oscillation of the specimen at two points, namely the amplitudes of oscillations of the free end of the specimen [W.sub.0] and in the clamp of the intermediate plate [W.sub.1]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2)

where coefficient P and the argument [alpha]x are determined by the equations

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (4)

The Krylov functions U, V, T, S were determined using the tables published in [8].

[FIGURE 1 OMITTED]

Analysis of the error of the values included in the equation for the determination of the stress shows that the main contribution to the error of calculation the stresses using this method is provided by the measurement of the amplitudes [W.sub.0] (2%), the elasticity modulus E (1%) and the density (0.7%) of the investigated materials.

The error in the determination of the values [alpha] and P, included in the equations (3), (4) has only a slight effect on the accuracy of the calculations. The total systematic error of the calculation of the stresses resulting from calculations using equation (2) is [+ or -] 2%.

The effective limit [[sigma].sub.-1] was determined by the graphic method on the basis of the fatigue curves for [10.sup.7] cycles.

The cyclic cracking resistance characteristics were estimated using the appearance of the fracture surface and the procedure used by the authors of [8, 9] where the fracture surface in the fatigue test was analysed by the scheme of fatigue failure of cylindrical specimens of ductile materials in bending (Fig. 2). The values of the stress intensity factor resulting in the start of the growth of a normal separation cracking the cylindrical specimens was determined, as in [9] using the equation

[K.sub.1S] = [[sigma].sub.a] [([[pi]/.sub.s]).sup.1/2], (5)

where [[sigma].sub.a] is the amplitude of the fracture stress, [l.sub.S] is the depth of the fatigue crack or depth of the defect at which fracture by normal separation starts.

[FIGURE 2 OMITTED]

In the study, the value [l.sub.S] was determined on the basis of the photographs of the fracture surfaces of the specimens after the fatigue test using a Superprobe 733 electron microscope.

3. Experimental results

The results of the determination of the mechanical properties in the tensile test and of the values of the elasticity modulus and the fatigue resistance in bending of the investigated materials are presented in Table 1. The fatigue curves obtained in bending of the alloys in the groups 1 2 are presented in Fig. 3.

Figures 4 and 5 shows the photographs of the fracture surfaces of the specimens of the investigated alloys after fatigue testing at different [[sigma].sub.-1].

4. Discussion of the experimental results

The values obtained for the elasticity modulus E (Table 1) show that the value of E for the Al-5Mg alloy after extrusion is almost identical with the value of E published for this alloy in the literature, i.e., 70 GPa. Additional alloying of the alloy with 0.3 wt.% Sc and 0.15 wt.% Zr results in a small increase of the elasticity modulus of this material. The value of E can be further increased by adding 0.2-0.5 wt.% Cr.

[FIGURE 3 OMITTED]

The elasticity modulus of the Al-9Zn-3Mg-2.3Cu alloy (No. 1, group 2, Table 1) is equal to 69.3 GPa, and additional alloying with 0.15 wt.% Zr [+ or -] 0.3 wt.% Sc increases the elasticity models to 70.5 GPa. Further alloying of the material was 0.3 wt.% Mn, 0.2 wt.% Cr, 0.2 wt.% Hf, and 0.2 wt.% Nb slightly reduces the value of the elasticity modulus E to 72-73 GPa.

However, the effect of additional alloying on the increase of the fatigue limit values and the complex of the mechanical properties in the tensile test is more distinctive than the effect on the Young modulus. It should be mentioned that the main role in increasing the yield and tensile strength values of the alloys of the group 1 and 2 is played by alloying with Sc, as confirmed previously in [1-5].

For the basic alloy of the first group Al-5Mg, the fatigue limit in cantilever bending with a frequency of 1600 Hz for [10.sup.7] cycles is 135 MPa (Fig. 3). This is close to the results obtained in [10-12].

According to the data published in [10], the fatigue limit of AMg5M alloy in bending a cantilever specimen with a diameter of 8 mm for 2 * [10.sup.7] cycles is 95 MPa, and according to the results of [10] the AMg6 alloy has the fatigue limit of 120 MPa for the same number of cycles. According to the results published in [11], the fatigue limit of 11 alloys, belonging to the Al-Mg system and obtained in fatigue testing at 5-108 cycles, changes in the range 120-152 MPa.

[FIGURE 4 OMITTED]

According to the data in Table 1, the addition of the complex (0.3 Sc-0.15Zr-0.2Cr) of the basic Al-5Mg alloy increases the fatigue limit to 210 MPa, which is 56% higher than the fatigue limit of the basic alloy in this investigation and greatly exceeds the value published in the literature for the alloys of this system.

The fatigue limits of the basic Al-9Zn-3Mg-2.3Cu alloy is 139 MPa. The addition of the complex (0.3Mn-0.3 Sc-0.50 Zr-0.2Cr) to the basic alloy Al 9Zn-3Mg-2.3Cu increases the fatigue limit to 280 MPa, reaches 100% higher than that of the basic alloy (Table 1, Fig. 3).

According to the data in [12], the fatigue limit in fatigue testing at [10.sup.7] cycles and R = 0 for the 7050 alloy (Al-6.27Zn-2.3Mg-2.3Cu-0.12Zr) changes from 130 to 150 MPa, and the fatigue limit of the 7178 alloy (Al-6.87 Zn-2.8 Mg-2.0Cu-0.23Cr) changes from 205 to 235 MPa.

The fracture surface of the specimens after the fatigue test was analysed on the specimens with relatively high and low amplitudes of cycling loading. The photographs of the fatigue fracture surfaces of the specimens of the melts of the groups 1 and 2 are shown partially in Fig. 4 and 5.

It has been established that the course of the fatigue test for the alloys 1 and 2 groups greatly differs.

The specimens produced from the alloys of the group 1 fractured at a small decrease of resonance frequency in the fatigue test which according to the test conditions indicates a high rate of failure. The fracture surface of the specimens of the alloys of the group 1 show that macrocracks and microcracks are situated normal to the longitudinal axis of the specimen after the fatigue test. The photographs of the fracture surfaces produced at a magnification of x300 show traces of propagation of semi-elliptical fatigue cracks in the form of zones with different microrelief on the fracture surface. In particular, in the specimens of the basic alloys (not alloyed with Sc and TM) cracks with a relatively coarse fracture surface propagate from the surface of the centre of the specimen, and in the specimens of the alloys alloyed additionally with Sc and Zr and with transition metals, the fracture surface is not so rough.

[FIGURE 5 OMITTED]

Examination at a magnification of 1000 shows that the coarse fracture surface is determined by the presence of relatively large structural element--the grains or particles with the size of approximately 20 [micro]n. At the same time, in the specimens produced from the alloys additionally alloyed with SC and transition metals, the size of these elements in the fracture surface is at least three times smaller, 5-10 [micro]m. In addition, the fracture surface of the specimens contained dimples typical of ductile failure.

The resultant fracture surfaces slightly differ from the scheme in Fig. 2 [9]. In particular, in the fatigue failure the specimens of the alloys of the Al-Mg system (Fig. 4), the zone 1 in it which, according to [9], the ductile material starts to fail under the angle of 45[degrees] to the longitudinal axis of the specimen, was not detected.

The failure of the specimen started from the structural elements with the size [l.sub.0] < [l.sub.S] (II), mostly in the direction normal to the axis of the specimens. Fracture surface analysis at different magnification shows that the fracture surface of the investigated alloys can be divided into four zones, as carried out in [6]. The size of the zone 1 is equal to the size of the fraction or microcrack on the surface of the specimen from which fatigue failure started at the depth [l.sub.0], the size of the zone 2 (III, according to the scheme in Fig. 2 according to [0]) is equal to the size of the part of the fatigue fracture surface containing mostly microseparation facets and situated normal to the axis of the specimen. The depth of the zone 1 was determined at a magnification of x 300 by analysis of the fracture surface of the large elements of the structure and the presence of these elements indicates the brittle mechanism of fracture of the operation of the microseparation mechanism.

The experimental results show that the specimens produced from the alloys of the group 2 (in contrast to the nature of fracture of the group 1 alloys) failed at a slow reduction of the resonance frequency of the fatigue test, i.e., at a low fracture rate.

The fatigue fracture surface of the specimens of the alloys of the group 2 contains a large proportion of fatigue fracture directed under the angle of 45[degrees] to the axis of the specimen. This appearance of fatigue fracture and the previously mentioned special features observed in the fatigue testing of the group 2 specimens, indicate that this material is capable of fracturing by the microshear mechanism under the effect of tangential stresses whose magnitude is maximum in the centre of the cross-section of the specimen.

The analysis of the fracture surfaces obtained in the fatigue testing of the specimens produced from the ingots of the group 2 (Fig. 5) shows that the specimens of the basic alloy (Al-9Zn-3Mg-2.3Cu) have larger grains and fail by brittle fracture under the effect of normal stresses at the crack tip (Fig. 5a). However, the zone 4 and also the area of slow crack growth in the zone of transition from zone 1 two zone 2 is characterised by the ductile failure by the micro sheer mechanism (Fig. 5e, f).

It should be mentioned that the appearance of the fracture surface (Fig. 5c, d) shows clearly that the start of fracture of the specimen of the alloy Al-9Zn-3Mg-2.3Cu-0.3 Mn-0.15Zr-0.3Sc-0.2Cr is associated with the failure of a large particles distributed in the vicinity of the surface of the specimen: the fatigue crack forms at fracture of these zone. In the alloys, containing SCu and Zr, these particles include the primary particle [Al.sub.3]([Sc.sub.1-x][Zr.sub.x]), formed during solidification of the alloy. This is confirmed by the photograph of the fatigue fracture surface made at a magnification of 2000 in x-ray radiation of Sc (Fig. 5b). The photograph shows that scandium is found in the particles distributed in the fatigue fracture. According to the results it can be concluded, firstly, that the cyclic strength of the investigated alloys can be increased by producing a fine-grained structure.

It may also be concluded that the 'weak' area in the alloyed alloys of the second group is not the grain size but the primary particles with the size of 7-12 [micro]m. The reduction of the size of this particles increases the cyclic strength.

Analysis of the fatigue fractures of the investigated materials gave three values of the crack depth in the majority of the investigated specimens. However, on the first two values of the crack depth can be substituted in to equation (4) and evaluate the cycling cracking resistance of the investigated materials.

The third value of the crack depth [l.sub.S] cannot be substituted in to equation (4) because at these crack depth (from 0.3 to 0.6 of the diameter of the specimen) it is very difficult to determine the stress in the upper part of the crack in fatigue testing in the resonance conditions.

The results of determination of the cycling cracking resistance of the investigated material is using equation (4) are presented in the final columns of Table 2.

The analysis of the data shows that the microstructural cycling cracking resistance [K.sup.0.sub.1S] of the Al-5Mg alloy is an average 1.27 [MPa*[m.sup.050] the initial cycling cracking resistance [K.sup.11.sub.1S] [6] is on average 2.31 [MPa * m.sup.0.50]. Alloying of the basic material with a complex (0.2Cr-0.3Sc-0.15 Zr) results in a reduction of the mean size of the elements of the microstructure to the mean values [K.sup.0.sub.1S]--1.0 [MPa * m.sup.0.5] and also [K.sup.11.sub.1S]--5.37 [MPa * m.sup.0.5].

For the alloys of the Al-Zn-Mg-Cu system: for the basic alloy [K.sup.0.sub.1S]--232 [MPa * m.sup.0.5] and [K.sup.11.sub.1S]--5.17 [MPa * m.sup.0.5]; for the alloy alloyed with the complex (0.3 Mn-0.15Zr-0.3Sc-0.2Cr), [K.sup.0.sub.1S]--1.8 [MPa * m.sup.0.5] and--6.6 [MPa * m.sup.0.5].

The previously stated values of the cyclic cracking resistance [K.sup.0.sub.1S] and [K.sup.11.sub.1S] could not be found in the literature for aluminium alloys, but the results of the analysis of the fatigue fractures indicate that the value [K.sub.1S.sup.11] is close to the threshold stress intensity factor Kth at which the crack does not grow during the period of [10.sup.6] cycles and the increase of the value by 3% results in growth of the crack at the rate lower than 3 * [10.sup.-10]. m/cycle [13]. In this case, the value Kth is determined from the equation

[DELTA]Kth = [[sigma].sub.1][[sigma].sub.-1][([pi][a.sub.c]).sup.1/2],

where [y.sub.1] = 0.65 for a semi-circular surface crack, [[sigma].sub.-1] is the fatigue strength for finite life, [a.sub.c] is the critical size of the microcracks.

Table 2 shows the results obtained using the scheme in Fig. 2 [9], taking into account special features of the fractures of the investigated materials in accordance with [6], in the measurement of the fatigue fracture zones on the photographs of the fracture surfaces of the specimens. It was shown in [10] that for the AMg6 aluminium alloy in cycling cracking resistance test with off-centre loading Kth = 2.55 [MPa * m.sup.0.5]. At the same time, the results of investigations with the symmetric tension-compression of the AMg6N alloy with the loading frequency of 3 kHz [14] gave Kth = 0.98 [MPa * m.sup.0.5].

Thus, the cycling cracking resistance of the group 1 alloys, alloyed with Cr, Sc and Zr, is at least twice the cycling resistance Kth published in the literature. The study [13] presents the data on the threshold stress intensity factors of the alloys of the system 7XXX, obtained in bend testing. In this case, Kth= 4.8 [MPa *m.sup.0.5]. In [4] for the 7010 alloy (according to the US standard) Kth = 5.0-6.2 [MPa * m.sup.0.5]. This result corresponds to the values of [K.sub.1S.sup.11] obtained in the present investigation for the group 2 alloys (Table 2).

5. Conclusions

The alloying of the alloys of the system Al-5 Mg with (0.3Sc-0.15Zr-0.2Cr) and the corresponding conditions of production of the components from these alloys results in a small increase of the elasticity modulus, a 56% increase of fatigue limit, and also in more than doubling of the cyclic cracking resistance. The increase of the mechanical characteristics is accompanied by a decrease of the mean size of the mean size of the cleavage facets on the fatigue fracture surface by a factor of 3 in comparison with the basic material. Alloying of the alloys of the Al-Zn-Mg-Cu system with the complex (0.3 Mn-0.15 Zr-0.3Sc-0.2Cr) also results in a small increase of the elasticity modulus and in doubling of the fatigue limit. However, in this case, the cyclic cracking resistance of the alloy does not increase greatly and alloying with this complex of the elements.

The fatigue resistance of the investigated alloys, alloyed with Sc and transition metals can evidently be increased by reducing the size of the primary particles of the intermetallic compounds by increasing the cooling rate of the melt as a result of the optimum ratio of the alloying elements.

References

(1.) Yu. V. Milman, et al., Materials Science Forum 'Aluminium Alloys. Their Physical and Mechanical Properties' (Switzerland: TransTech Publications: 2000), vol. 331-337, p. 1107.

(2.) Yu. V. Milman, Advanced materials and technologies, Akademperiodika, Kiev, 2003.

(3.) C. B. Fuller, et al., Mater. Sci. Eng. A, 338, No. 2: 8 (2002).

(4.) M. N. Desmukh, et al., Mater. Sci. Eng. A, 435, No. 36: 318 (2006).

(5.) C. Watanabe, et al., Int. J. Fatigue, 30: 635 (2008).

(6.) V. A. Kuz'menko, Sound and ultradouns oscillations in dynamic testing of materials, Acad. Sciences, Kiev, 1963.

(7.) Yu. F. Lugovskoi, Probl. Spets. Electrometall., No. 4, 61, 1987.

(8.) G. S. Pisarenko, et al., A handbook of strength of materials, Naukova dumka, Kiev, 1988.

(9.) V. S. Ivanova, Fracture of metals, Metallurgiya, Moscow, 1979.

(10.) V. V. Panasyuk (editor), Fracture mechanics and strength of materials, L'viv, SPOLOM, vol. 9, 2007.

(11.) V. T. Troshchenko and A. Sosnovskii, Fatigue strength of metals and alloys, vol. 2, Naukova dumka, Kiev, 1987.

(12.) ASM Specialty Handbook: Aluminum and Aluminum Alloys (Ed. J. R. Davis) (Materials Park, OH: ASM International: 1993).

(13.) S. Ya. Yarema, Fiz. Khim. Mekh. Mater., No. 6, 47 (1975).

(14.) T. Yu.. Yakovleva, Local plastic deformation and fatiuge of metals, Naukova dumka, Kiev, 2003.

Yu.V. Mil'man, Yu.F. Lugovs'koi, A.I. Sirko, A.O. Sharovskii and A.V. Samelyuk

I.M. Frantsevich Institute of Materials Science, bul. Krizhanovskogo 3, 03142 Kiev, Ukraine
Table 1. Mechanical properties and density ([rho]) of the investigated
alloys (systems Al-Mg and Al-Zn-Mg-Cu)

                                                       Mechanical
                                       [rho] *         properties
                                       [10.sup.3],     E, GPa
No.   Chemical composition, wt.%       kg/[m.sup.3]

                                                       Group 1

1     Al-5Mg                           2.67            70.2
2     Al-5Mg-0.3Sc-0.15Zr              2.67            72.4
3     Al-5Mg-0.3Sc-0.15Zr-0.2Cr        2.67            72.3
4     Al-5Mg-0.3Sc-0.15Zr-0.5Cr        2.67            73.9

                                                       Group 2

1     Al-9Zn-3Mg-2.3Cu                 2.9             69.3
2     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   2.9             74.5
      0.3Sc
3     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   2.9             72.2
      0.3Sc-0.5Hf
4     Al-9Zn-3Mg-2.3Cu-0.3Mn-          2.9             73.4
      0.15Zr-0.3Sc-0.15Nb
5     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   2.9             73.0
      0.3Sc-0.2Cr
                                       Mechanical properties
                                       [[sigma].sub.B]
No.   Chemical composition, wt.%       Mpa

1     Al-5Mg                           298
2     Al-5Mg-0.3Sc-0.15Zr              480
3     Al-5Mg-0.3Sc-0.15Zr-0.2Cr        489
4     Al-5Mg-0.3Sc-0.15Zr-0.5Cr        521

1     Al-9Zn-3Mg-2.3Cu                 619
2     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   789
      0.3Sc
3     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   810
      0.3Sc-0.5Hf
4     Al-9Zn-3Mg-2.3Cu-0.3Mn-          824
      0.15Zr-0.3Sc-0.15Nb
5     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   804
      0.3Sc-0.2Cr

                                       Mechanical properties
                                       [[sigma].sub.0.2']   [delta], %
No.   Chemical composition, wt.%       MPa

1     Al-5Mg                           167                  30.5
2     Al-5Mg-0.3Sc-0.15Zr              375                  12.8
3     Al-5Mg-0.3Sc-0.15Zr-0.2Cr        376                  12.7
4     Al-5Mg-0.3Sc-0.15Zr-0.5Cr        397                  13.1

1     Al-9Zn-3Mg-2.3Cu                 530                  20.4
2     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   696                  12.3
      0.3Sc
3     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   700                  14.1
      0.3Sc-0.5Hf
4     Al-9Zn-3Mg-2.3Cu-0.3Mn-          722                  11.4
      0.15Zr-0.3Sc-0.15Nb
5     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   725                  10.2
      0.3Sc-0.2Cr

                                       Mechanical properties
                                       [[sigma].sub.17']
No.   Chemical composition, wt.%       MPa

1     Al-5Mg                           135
2     Al-5Mg-0.3Sc-0.15Zr               --
3     Al-5Mg-0.3Sc-0.15Zr-0.2Cr        210
4     Al-5Mg-0.3Sc-0.15Zr-0.5Cr         --

1     Al-9Zn-3Mg-2.3Cu                 139
2     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-    --
      0.3Sc
3     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-    --
      0.3Sc-0.5Hf
4     Al-9Zn-3Mg-2.3Cu-0.3Mn-           --
      0.15Zr-0.3Sc-0.15Nb
5     Al-9Zn-3Mg-2.3Cu-0.3Mn-0.15Zr-   280
      0.3Sc-0.2Cr

Table 2. Fatigue limit, the depths of fatigue cracks and cyclic
cracking resistance of the investigated alloys (systems Al-Mg,
Al-Zn-Mg-Cu)

No.   Chemical                   Endurance               Size of areas
      composition, wt.%          [[sigma].sub.-1]        acc. to [6,9]
                                 at the number of        measured on SEM
                                 cycles N                fracto-graphs
                                 [[sigma].sub.-1']       [l.sub.0,]
                                 MPa    N                [micro]m

1                                193    4.[10.sup.5]     15
      Al-5Mg
2                                154    4.8.[10.sup.6]   20
3                                261    4.[10.sup.5]     5
      Al-5Mg-0.3Sc-0.15Zr-
      0.2Cr
4                                210    3.[10.sup.6]     7
5                                193    [10.sup.6]       50
      Al-9Zn-3Mg-2.3Cu
6                                165    4.8.[10.sup.6]   60
7                                352    5.8.[10.sup.5]   12
      Al-9Zn-3Mg-2.3Cu-
      0.3Mn-0.15Zr-0.3Sc-0.2Cr
8                                310    9.7.[10.sup.5]   7

                                 Size of areas acc. to [6,9]
No.   Chemical                   measured on SEM fracto-graphs
      composition, wt.%
                                 [l.sub.s11]   [l.sub.s']
                                 [micro]m       [micro]m

1                                 50              1000
      Al-5Mg
2                                 66               875
3                                 83              1500
      Al-5Mg-0.3Sc-0.15Zr-
      0.2Cr
4                                308              1250
5                                330              2000
      Al-9Zn-3Mg-2.3Cu
6                                200                --
7                                150              1200
      Al-9Zn-3Mg-2.3Cu-
      0.3Mn-0.15Zr-0.3Sc-0.2Cr
8                                100              1300

No.   Chemical                   Cyclic cracking resistance
      composition, wt.%
                                 [K.sup.0.sub.1S]'
                                 MPa [M.sup.05]

1                                1.32
      Al-5Mg
2                                1.22
3                                1.03
      Al-5Mg-0.3Sc-0.15Zr-
      0.2Cr
4                                0.98
5                                2.41
      Al-9Zn-3Mg-2.3Cu
6                                2.26
7                                2.16
      Al-9Zn-3Mg-2.3Cu-
      0.3Mn-0.15Zr-0.3Sc-0.2Cr
8                                1.45

No.   Chemical                   Cyclic cracking resistance
      composition, wt.%
                                 [K.sup.11.sub.1s']
                                   MPa [M.sup.05]

1                                       2.41
      Al-5Mg
2                                       2.21
3                                       4.22
      Al-5Mg-0.3Sc-0.15Zr-
      0.2Cr
4                                       6.52
5                                       6.21
      Al-9Zn-3Mg-2.3Cu
6                                       4.13
7                                       7.64
      Al-9Zn-3Mg-2.3Cu-
      0.3Mn-0.15Zr-0.3Sc-0.2Cr
8                                       5.49
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Title Annotation:PHYSICS OF STRENGTH AND PLASTICITY
Author:Mil'man, Yu.V.; Lugovs'koi, Yu.F.; Sirko, A.I.; Sharovskii, A.O.; Samelyuk, A.V.
Publication:Physics of Metals and Advanced Technologies
Article Type:Report
Geographic Code:4EXUR
Date:Jan 1, 2010
Words:4602
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